Energy Efficient USB 2.0 Proposed, Denied Entry to Windows 7

In August, Intel announced an addendum to the USB 2.0 specification, intended to define several energy efficiency extensions. At November's WinHEC event, Microsoft officially noted that the extensions won't be supported by Windows 7. What gives?

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In August, Intel announced an addendum to the USB 2.0 specification, intended to define several energy efficiency extensions. At November's WinHEC event, Microsoft officially noted that the extensions won't be supported by Windows 7. What gives?

The proposal would extend the USB Enhanced Host Controller Interface spec (EHCI is the technical term for the USB 2.0 controller on your PC, not to be confused with a Universal Host Controller Interface or UHCI, the USB 1.1 controller). The new power saving features center around addition of link power management, by which the controller on your PC and a gadget can negotiate a lower power connection during idle times. Power users might have noticed an option in Vista's control panel to adjust a similar setting for PCI Express devices, called Link State Power Management. And there's a movement to monitor Ethernet connections, for additional power savings. Why won't MS support it in USB too? After the jump, a brief technical explanation of the changes...and why MS won't support them.

Link Power Management: Defines the mechanisms by which software can discover and use Link Power Management (LPM) support, to be offered by next-generation EHCI host controllers and USB devices. The implementation leverages much of the existing Suspend/Resume infrastructure to streamline software support for this key energy efficiency feature.

Per-Port Change Events: Addresses an issue where system software is currently required to read and parse the port status registers (PORTSC) for all enabled root ports upon every port change event (interrupt). This is particularly important as LPM state transitions will be exposed to software as port change events, noting the software overhead combined with the potential increase in event frequency could negatively impact OS responsiveness and system-level energy efficiency.

The PCI interface is a parallel bus architecture. When no data is sent, there is no activity on the bus and thus, no power consumption. PCI Express is based on a series of high-speed serial links. Even when no data is sent, serial links continuously exchange data to remain synchronized. This additional activity causes PCIe to consume a great deal more power than PCI.

Active State Power Management (ASPM) lets a PC incrementally reduce power for individual serial links as they become less active. Windows Vista enables or disables ASPM based on the overall system power policy, the hardware's capabilities, and the link's latency. But the hardware performs the actual power management; it needs to be able to report its capabilities to Windows and actively manage itself.

USB is, of course, serial (it's the universal serial bus, remember?). So it works in a similar way, continually remaining synchronized and sucking power. To enable this type of power management will require a new generation of USB devices, since current EHCI controllers simply don't have this capability. Bottom line, it's anyone's guess when Intel will start shipping hardware that supports it. And right now, both Intel and Microsoft are focused on finalizing he spec and support for USB 3.0 devices. Which would you rather see?

Jeremy Kaplan is a former executive editor for PC Magazine and co-host of the Fastest Geek competition. He also served as Editor of GoodCleanTech.com.
Kaplan helped to determine overall editorial direction, managed staff, and shaped the editorial calendar. Prior to this, Kaplan succumbed to his inner geek, launching the spin-off publication ExtremeTech Magazine. During this time, he helped popularize the Fastest Geek competition, where contestants compete to assemble a computer from parts as quickly as possible. Kaplan graduated from Vassar College in 1996, majoring in both...
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